Test probes are typically used to provide an electrical path between a test system and circuits on a wafer or die, thereby permitting the testing and validation of the circuits thereon, before they are packaged. The probes are often long and narrow, and are fit within a probe card that holds the probes in place. Test probes are typically formed from electrically conductive materials that have adequate mechanical properties to survive the repeated uses and elevated temperatures that the probe tip sees when in service. As a result, lower strength metals with low resistivity such as pure gold, silver, and copper are typically not used exclusively as advanced test probe materials.
Test probes are becoming more difficult to manufacture due to the continued scaling of Moore's law for first level interconnect pitch and the complexity of electrical and mechanical requirements for high volume manufacturing testing. Ohmic heating of test probes during large current delivery leads to irreversible deformation under load. As a result, low resistance materials are desirable to minimize the self-heating. Cycling during high volume manufacturing test socketing may lead to early fatigue failures of probes. As a result, high strength materials are sought for the probe. However, materials with both low resistance (high conductivity) and high strength for use as a probe are not common.
Among the types of probes include MEMS (micro-electro-mechanical systems) probes, which can be formed to combine the properties of good electrical conduction (low ohmic heating) and good springs (strong and tough) into a single probe design with composite materials. Such MEMS probes, however, are often complex and expensive to manufacture. Another probe type is a buckling beam type, which is manufactured using drawn wire and a cold-work forming process. Such a process is generally limited to metals that can be easily cold-formed, which often do not provide the sought after conductivity and mechanical properties characteristics for advanced test probes.
Tungsten (W) and its alloys offer good properties for use as a test probe, due to its relatively low resistivity and high mechanical strength. It has been largely unavailable for advanced probes (for example, those with curved structures and tight tolerances) due to manufacturing challenges, as it cannot be plated or deposited using thin film CVD deposition to thicknesses greater than about 2 microns, and the thick sheets (about 50 microns) cannot be etched to form straight sidewalls. Devices with C4 bumps generally require high density array probing and complicated vertical designs with elaborate curves and tight tolerances. Drawn tungsten wire cannot be formed into such complicated bends and geometries with tight tolerances.